1. Field of the Invention
The present invention relates to a communications system wherein system components communicate via a communications bus. More particularly, the invention relates to interface modules for such system components and to a method of protecting such interfaces.
2. Description of the Related Art
An exemplary communications system comprises a computer and one or more peripheral devices. The peripheral devices can include, for example, external disk drives, printers or data communications devices. The computer and the peripheral devices communicate via a communications bus which is, for example, a bus in accordance with a Small Computer System Interface (SCSI) specification. The SCSI specification defines a bus that comprises 54 lines and supports bidirectional data transmission in two electrical specifications, single-ended and low-voltage differential. Further details of the SCSI bus are described in a book entitled "SCSI Understanding the Small Computer System Interface," PTR Prentice Hall, Englewood Cliffs, N.J. 07632, ISBN 0-13-796855-8.
The computer and the peripheral devices comprise interface modules which, inter alia, organize communications between the computer and the peripheral devices in accordance with a defined communications protocol.
Internally, the interface modules comprise integrated circuits which are, for example, made with a semiconductor process technology using a line width of 0.45 microns or below. These circuits are intended to be in communication with a 3-volts bus. Manufacturers of integrated circuits desire to add more integrated circuits into one chip to reduce the complexity of printed circuit boards. To control the power dissipation problem caused by densely packed circuits, the manufacturers try at the same time to reduce the power supply voltage necessary to operate the integrated circuits, for example, to about 3.3 volts and below. This requires a reduction of the threshold voltages of the field effect transistors used in such integrated circuits. By reducing the thickness of the gate oxide from about 120 angstrom (.ANG.) to about 70 .ANG., the gate oxide breakdown voltage of a transistors has to be reduced from about 6-7 volts to about 4.5 volts.
The reduced gate oxide breakdown voltage poses a potential risk for an integrated circuit included in an inactive device when this inactive device is connected to an active system ("hot plugging"). Under such a condition, a voltage applied to transistors of the integrated circuit may exceed the breakdown voltage and destroy an integrated circuit.